1. Field of the Invention
The present invention relates to an integrated sensor for a scanning probe microscope (SPM).
2. Description of the Related Art
From the latter half of 1980 onwards, proposals have been made to provide, as one of scanning probe microscopes, the optical microscope which is called a scanning near-field optical microscope (SNOM). The SNOM achieves a high resolution above the diffraction limitation, using an evanescent wave, which exists only in a region smaller in size than the optical wavelength and does not propagate in a free space.
The SNOM obtains an image of a sample by holding a probe with a very small aperture at its tip over a surface of the sample at a distance less than the optical wavelength and by mapping an intensity of light passing through the aperture.
Several types of the SNOM have been proposed and are broadly classified into a collection type and an emission type. In the collection-type SNOM, a SNOM image is obtained by detecting an evanescent wave, by means of the probe, which emerges on the sample surface illuminated with light coming from below. In the emission-type SNOM, on the other hand, a SNOM image is obtained by introducing light into the probe to emerge as an evanescent wave at the tip of the probe, by contacting the evanescent wave with a sample to convert it to propagating light, and by detecting the propagating light with a light detector positioned beneath the sample. An emission-type SNOM is disclosed, for example, in JPN PAT APPLN KOKAI PUBLICATION 4-291310.
Recently, N. F. van Hulst, et al. have proposed a method for effecting simultaneous measurement on the SNOM and AFM (atomic force microscope) using a cantilever in place of an optical fiber probe. This type of SNOM is disclosed, for example, N. F. van Hulst, M. H. P. Moers, O. F. J. Noordam, R. G. Tack, F. B. Segerink and B. Bolger, "Near-field optical microscope using a silicon-nitride probe", Appl. Phys. Lett. 62, 461-463, (1993).
As for the AFM, a sharp projection or probe, which is formed at a free end of a cantilever, is positioned close to a sample surface and is scanned across the sample surface. During the scan, a displacement of the cantilever (strictly speaking, of the probe), which is caused by an interaction force between atoms of the probe tip and the sample surface, is electrically or optically measured. Height information at each point on the sample surface is obtained on the basis of the displacement, and a three-dimensional image representing a configuration of the sample surface is formed by processing the height information synchronizing with positional data of the probe.
In the AFM, a sensor for measuring the displacement of the cantilever is generally provided separately from the cantilever. In recent years, an integrated sensor for an AFM, in which a cantilever has the function of measuring the displacement of itself, has been proposed. The integrated AFM sensor is disclosed, for example, in M. Tortonese, H. Yamada, R. C. Barrett and C. F. Quate, "Atomic force microscopy using a piezoresistive cantilever", Transducers and Sensor '91 and in PCT application WO92/12398.
The integrated AFM sensor uses a piezoresistive effect in its measurement principle. The integrated sensor has a cantilever in which a resistive layer is provided, and a constant voltage is applied to the resistive layer. When the probe tip (the tip end of the cantilever) is held over a sample, the cantilever is distorted due to an interaction between the probe and the sample. The resistance of the resistive layer varies in accordance with the magnitude of the distortion, such that an electric current flowing in the resistive layer changes. That is, the electric current flowing in the resistive layer varies in accordance with an amount of distortion or displacement of the cantilever. Consequently, the amount of the displacement of the cantilever is measured by detecting the changes of the electric current flowing in the resistive layer.
Being simple and compact in arrangement, the integrated AFM sensor is expected to be used as a so-called stand-alone type AFM, which scans a cantilever side. In the conventional AFM, since the position of the probe with respect to the sample is varied by moving the sample in X and Y directions, the size of the sample is restricted at maximum to the nearest few centimeters. The stand-alone type AFM has no such restriction and is able to measure a large sample.
Here, an explanation will be given below about the integrated AFM sensor and the drawings. In the beginning its manufacturing method will be described, referring to FIGS. 12A to 12D. As shown in FIG. 12A, a starting wafer 100, e.g., a bonded wafer, in which a silicon layer 114 is formed over a silicon wafer 110 with an isolation layer 112 of a silicon oxide provided therebetween, is prepared. Boron (B) ions are implanted in the silicon layer 114 at the surface, and, after being patterned to a configuration as shown in FIG. 12D, a resultant structure is covered with a silicon oxide layer 118. Holes for bonding are provided on the fixed end side of a cantilever and aluminum (Al) is sputtered there to provide electrodes 120. Further, a resist layer 122 is formed on a lower surface side of the silicon wafer 112 and patterned to provide an opening as shown in FIG. 12B. After a heating treatment step for providing ohmic contacts, the silicon wafer is etched by a wet type anisotropic etching with the resist layer 122 as a mask till the isolation layer 112 appears. Finally, the isolation layer 112 is etched with a hydrofluoric acid to provide a cantilever 124, so that an integrated sensor for an AFM is completed. A side cross sectional view and a top view of the sensor is shown in FIG. 12C and FIG. 12D, respectively.
FIG. 13 shows a circuit arrangement for effecting displacement measurement using the integrated AFM sensor. As shown in FIG. 13, a constant voltage supply 126 and an operational amplifier 128 for current measurement are connected to corresponding terminals 120 of the piezoresistive cantilever 124. With the potential of the constant voltage supply 126 set at a +5 volt, a potential on the terminal 120 of the piezoresistive cantilever on the upper side in FIG. 13 stays at a +5 volt. The other terminal 120 of the piezoresistive cantilever 124 is maintained at a GND potential since the non-inverting input terminal of the operational amplifier is set at a GND potential.
When the free end of the cantilever 124 is approached so close to the sample to cause an interaction between the atoms on the tip of the cantilever 124 and the surface of the sample, the cantilever 124 is displaced and hence the resistance of the piezoresistive layer 116 varies accordingly. As a result, the displacement of the cantilever 124 is detected as a current signal flowing between the two electrodes 120.
Recently, an integrated sensor for an SPM, which also detects an amount of torsion (LFM signal) of the cantilever 124, has also been proposed. The integrated sensor like this is disclosed, for example, in U.S. Pat. No. 5,386,720.
Also, a strain sensor using a piezoresistive layer can be manufactured by a silicon planer technique. The strain sensor is simple in arrangement and small in size, and is, therefore, suited to be an integrated part of the cantilever. Since the strain sensor is composed of an electric current element, an electric current flows through the sensor during AFM measurement to detect displacement of the cantilever. As a result, heat is generated from the piezoresistive layer and thermal noise mixes into a displacement signal, i.e., into an electric current signal from the piezoresistive layer. Thus, the S/N of the detected signal is degenerated, so that the resolution and reliability upon AFM measurement is declined.
Although the integrated AFM sensor as proposed by M. Tortonese, et al. has a strain sensor as an integrated part of a cantilever, a composite structure, in which another sensor such as a light sensor, a temperature sensor, a magnetic sensor, etc., may be an integrated part of the cantilever in addition to a the strain sensor, may be considered. If, however, plurality of sensors are simply combined into the composite structure, there is a fear that output signals of the sensors would be mixed with each other or mutually affected so as to reduce the detection sensitivity of the signals.
In the case where the light sensor disclosed in U.S. Pat. No. 5,294,790 is representative of a light sensor for SNOM measurement provided on the integrated AFM sensor in actual practice, the strain sensor evolves heat due to a continuous flow of an electric current during measurement, so as to cause a raise in temperature of the light sensor, and to give influence to a dark current of the light sensor. In the situation that a photodiode is used for the light sensor, an ambient temperature raise by 5.degree. to 10.degree. C. causes a double dark current flow. An increase in dark current leads to a decline in S/N and in sensitivity of the light sensor. As a result, a minimal amount of light resolvable by the light sensor rises, and sensitivity of SNOM measurement decreases accordingly.
In the case where the strain sensor and light sensor are mounted onto one cantilever, a problem occurs due to an electric contact resulting from the voltages applied to the respective sensors. In a practical case, dark currents originating from voltages on a strain sensor and light sensor are mixed as noise into signals of the light sensor and strain sensor or a variation in a reference potential on the light sensor will arise. It is, therefore, not possible to achieve AFM measurement, as well as SNOM measurement, with improved sensitivity.